Editor's note: This is the first part of a series that has run in the past under the overall title of The Prometheus Trap. This series deals with the differences between Japan and the United States in dealing with the Fukushima nuclear accident of 2011. The series will appear on Mondays, Wednesdays and Fridays.

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Between late on March 14, 2011, and early the next morning, a top secret diplomatic cable arrived at the Foreign Ministry.

Sent three days after the accident at the Fukushima No. 1 nuclear power plant, the document detailed the major concerns that Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, had voiced to Japan's ambassador to the United States, Ichiro Fujisaki. At the time, the Japanese government had still not decided to use Self-Defense Force helicopters to dump water into the crippled nuclear plant.

Fujisaki, 65, sent the cable electronically under the highest level of confidentiality. Information included on the cable indicated which government agencies had the necessary clearance to read it and how long the document was to be kept. When it was printed, watermarks on the document identified the government agency it was intended for as well as the document number.

Because civil servants can face criminal charges for disclosing state secrets, no government source would publicly admit the cable existed.

Even after Fujisaki stepped down as ambassador in November last year, he denied the existence of such a cable when questioned by The Asahi Shimbun. And although Naoto Kan as prime minister at the time would have seen the cable, he said, "I do not remember" when asked about it.

However, a number of individuals who actually read the cable confirmed the contents of the document, which outlined the general frustration the United States felt over the inaction of the Japanese government.

Mullen frequently visited the White House for direct meetings with U.S. President Barack Obama. The cable depicts Mullen as raising serious doubts about how Japan was dealing with the situation because the overall view in Washington was that the response to the accident was being left in the hands of Tokyo Electric Power Co., the operator of the Fukushima plant.

On the morning of March 14, 2011, an explosion occurred at the No. 3 reactor following a similar explosion days earlier at the No. 1 reactor. In the afternoon, core pressure at the No. 2 reactor also began to rise as its cooling functions deteriorated.

However, Mullen's major concern was with the situation at the No. 4 reactor. He is reported as saying that the SDF should be used to cool the reactors.

"The U.S. military believes the No. 4 reactor is in danger. It feels every step should be taken to cool the reactor, including using the SDF," the cable said.

The No. 4 reactor had 1,535 nuclear fuel rods in its storage pool, a much greater number than the other three reactors. A meltdown would occur once the pool was empty of water, releasing huge volumes of radiation into the atmosphere and affecting not just Fukushima, but potentially all of Japan.

"The United States has made various preparations to deal with the nuclear accident. The president is also very concerned," the cable went on to say.

By invoking the White House, Mullen impressed upon Japanese Embassy officials that not just the U.S. military but the entire U.S. government was worried about how the situation was developing.

The Foreign Ministry distributed the cable to Kan and other central government ministries, with limits on who could access it.

A few hours after the cable reached Tokyo, at about 6 a.m. on March 15, Mullen's fear--an explosion at the No. 4 reactor--materialized. The pressure gauge at the No. 2 reactor also showed abnormal readings.

At 7 a.m., a situation arose which the U.S. military is still keeping quiet about.

At Yokosuka Naval Base, which lies about 300 kilometers from the Fukushima No. 1 nuclear power plant, alarms went off indicating an increase in radiation levels. All women and children on the base were immediately ordered to evacuate.

Because the U.S. Navy has nuclear-powered aircraft carriers, it has strict safety standards for radiation levels. The alarm was likely triggered by radiation that had reached the base from Fukushima.

U.S. government officials who were notified became very concerned because of the possibility that the Yokosuka Naval Base, considered of major strategic importance in East Asia, would become inoperable if the situation at the Fukushima plant worsened.

Such developments, coupled with Japan's failure to indicate it was dealing seriously with the situation, led Washington to take matters into its own hands.

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